Approximately 360 million years ago, flying insects reached massive sizes (by today's standards). Why are modern-day insects so small compared to their ancestors?

Answer: Due to evolution and selection.

Explanation:

In the past, the amount of oxygen in the atmosphere was very high, this allowed the support of large insects. As time passed, the amount of oxygen in Earth became lower, and small insects were selected because they were the only ones who can survive in this new condition.